Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes Confined in Mesoporous Silica: Tuning Transition States Towards Z‐Selective Ring‐Opening Cross‐Metathesis

Abstract

We recently reported a method for selective macro(mono)cyclization of dienes utilizing catalysts confined inside the pores of mesoporous silica, which we believe occurs due to suppression of oligomerization due to pore size. We hypothesized, however, that the system of cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) catalysts immobilized selectively inside the mesopores of silica materials could address much more subtle selectivity differences, such as E/Z selectivity in ring-opening/cross-metathesis (ROCM). Upon investigation, we observed that surface-bound cationic molybdenum imido alkylidene NHC catalysts indeed display an increased Z-selectivity, especially during the early stages of the reaction. This effect was present when the catalyst was confined inside a pore, as well as when the catalyst was bound to nonporous silica, which led us to conclude it is an effect caused by the catalyst being bound directly to the surface of a silica material where the proximity of the catalyst to the surface governs the transition state. Kinetic investigations revealed that significant post-metathesis olefin isomerization occurs, the amount of which seems to be governed by the rate of diffusion of the product away from the active catalyst, with smaller pore sizes resulting in higher Z-selectivity at higher conversion, attributable to faster diffusion of the product out of the pore than diffusion back into the pore.

Macrocyclization of Dienes under Confinement with Cationic Tungsten Imido/Oxo Alkylidene N‐Heterocyclic Carbene Complexes

“…Macrocyclization and Z-selectivity are highest with the smallest pores. The results presented here are in line with those obtained previously both in the macrocyclization and ring-opening crossmetathesis (ROCM) [6] with cationic molybdenum imido alkylidene NHC complexes under confinement and illustrate the generality of the confinement approach outlined here.…”

“…Even though the specific surface area of OMS 33Å is more than twice as large as that of OMS 68Å , the catalyst loadings on OMS 68Å are significantly larger. This effect may be rationalized by increasing diffusion limitations of W1 and W2 into the mesopores with decreasing pore sizes [6,9] …”

“…This effect may be rationalized by increasing diffusion limitations of W1 and W2 into the mesopores with decreasing pore sizes. [6,9] Four α,ω-dienes were used as substrates for MC; these were dec-9-en-1-yl undec-10-enoate 1, [5b,d,10] pentane-1,5-diyl bis(undec-10-enoate) 2, [5b,d,10] dodecane-1,12-diyl bis(undec-10enoate) 3, [5d] and tert-butyldimethyl(nonadeca-1,18-dien-10yloxy)silane 4 [5b,10] (Figure 1). To obtain the reference spectra of the macrocyclic products 5-8 (Figure 1), MC reactions were carried out at room temperature in CH 2 Cl 2 using the 2 ndgeneration Grubbs catalyst RuCl 2 (PCy 3 )(IMes)(CHPh).…”

“…We already reported on olefin metathesis‐based macrocyclization (MC) reactions under confinement using both, modified Grubbs‐Hoveyda‐type and cationic molybdenum imido alkylidene N ‐heterocyclic carbene (NHC) catalysts [5a–d,g,h,6] . These catalysts were selectively immobilized inside the mesopores of SBA‐15 and other ordered mesoporous silica (OMS) materials and allowed for macrocyclization (MC) reactions at concentrations of up to 100 mM with MC selectivities of up to 98 %.…”

Effect of liquid confinement on regioselectivity in the hydrosilylation of alkynes with cationic Rh(i) N-heterocyclic carbene catalysts

“…[16][17][18][19] Such pore-selective immobilization is an indispensable prerequisite since even small amounts of catalysts located outside the pores substantially obliterate connement effects. 20 One important nding was that the transition states of an organometallic catalyst can be tuned by locating the catalyst close to the surface, thereby generating steric constraints as non-covalent interactions that can be used to, e.g., enhance the Z-selectivity both in the macrocyclization of dienes 19 and in ring-opening cross metathesis reactions 21 that otherwise requires a careful ligand design. 22 Equally important, we found that the pore diameter as such plays a pivotal role.…”